1. Field of Invention
Exemplary aspects of the present invention relate to an image forming method and apparatus, such as a copier, a facsimile machine, and a printer, for enhancing the mobility of toner in a development device in an image forming operation according to an electrostatic copying process.
2. Description of Related Art
In recent years, automation and colorization in office environments have increasingly progressed. With this trend, in addition to existing opportunities to photocopy a text document, there have been increased opportunities to create a document including images, such as a graph, by using a personal computer, output the document from a printer, and photocopy the document in large numbers for a presentation, for example. Images output by the printer include a solid image, a line image, and a halftone image in many cases. With this tendency, the needs of the market for image quality have been changing. Further, a need for high reliability of images has been also increased.
In a development process, a developer used in an electrophotographic method, such as electrophotography, electrostatic recording, and electrostatic printing, is temporarily adhered to, for example, an image carrying member (typically a photoconductor) on which an electrostatic latent image is formed. Then, in a transfer process, the developer is transferred from the image carrying member onto a transfer medium, such as a transfer sheet.
Thereafter, in a fixing process, the developer is fixed on a recording medium. Related art developers used for developing the electrostatic latent image formed on the image carrying member include a two-component developer including carrier and toner, and a one-component developer not requiring the carrier and thus solely including the toner. The one-component developer is further divided into a one-component developer using a magnetic toner and a one-component developer using a nonmagnetic toner. The two-component developer deteriorates as toner particles adhere to surfaces of the carriers. Further, since only the toner is consumed in the two-component developer, the toner density in the developer decreases. To maintain a toner-to-carrier mixing ratio at a constant value, the development device is increased in size. Meanwhile, the one-component developer contributes to downsizing of the development device and is readily used under a variety of environments, such as a low-temperature and low-humidity environment and a high-temperature and high-humidity environment. Due to such advantages, the one-component developer is becoming the mainstream developer used in developing methods.
As described above, the one-component developer is divided into the magnetic one-component developer using the magnetic toner and the nonmagnetic one-component developer using the nonmagnetic toner. According to a magnetic one-component developer developing method using the magnetic one-component developer, the magnetic toner including a magnetic substance, such as magnetite, is held in a development sleeve which includes a magnetic field generating device, such as a magnet, and the toner is leveled into a thin layer by a layer thickness regulating member to be used in the development process. In recent years, the magnetic one-component developer developing method has been frequently used in small-size printers and the like. Meanwhile, according to a nonmagnetic one-component developer developing method using the nonmagnetic one-component developer, the toner does not have magnetic force. Thus, the toner is supplied to the development sleeve by pressing a toner supply roller, for example, onto the development sleeve, and the toner is electrostatically held thereon. Then, the toner is leveled into a thin layer by the layer thickness regulating member to be used in the development process. The nonmagnetic one-component developer does not include a colored magnetic substance and thus is compatible with the colorization. Further, since the development sleeve does not include a magnet, the nonmagnetic one-component developer developing method contributes to reduction in weight and cost of the development device. In recent years, therefore, the method has been practically used in a small-size full-color printer and the like.
However, the one-component developer developing method is still open to enhancements. According to the two-component developer developing method, the carrier is used as a medium for charging and conveying the toner. Further, the toner and the carrier have been sufficiently mixed in the development device before being conveyed to the development sleeve to be used in the development process. Therefore, the toner can be stably charged and conveyed even after a relatively long-time use. Furthermore, the method can be readily used in a high-speed development device. Meanwhile, the one-component developer developing method does not include the medium for stably charging and conveying the toner, such as the carrier. Therefore, operations of charging and conveying the toner tend to be improperly performed due to the long-time use of the toner and the increase in the operation speed of the development device.
In particular, in the nonmagnetic one-component developer developing method, the toner is conveyed onto the development sleeve and is leveled by the layer thickness regulating member into a thin layer to be used in the development process. In this process, the toner is contacted with the development sleeve and is friction-charged by a friction charging member, such as the layer thickness regulating member, in a substantially short time. Thus, insufficient charging and inverse charging of the toner tend to occur more frequently than in the two-component developer developing method which uses the carrier. Further, in the nonmagnetic one-component developer developing method, the toner is conveyed by at least one toner conveying member, and the thus conveyed toner is used to develop the electrostatic latent image formed on the image carrying member. In this process, the toner layer formed on a surface of the toner conveying member needs to be as thin as possible, and the toner in the thinned layer is applied with pressing force by the layer thickness regulating member. As a result, an external additive applied to surfaces of toner particles is buried deep into the toner particles, and chargeability and mobility of the toner is substantially decreased.
In light of the above, a related art forming apparatus includes, near a toner supply tank and a supply port of a toner hopper, a magnet roller serving as a supply roller and a scraper serving as a layer thickness regulating device. The supply roller is rotated in both forward and inverse directions by a supply roller driving device. When a rotation angle of the supply roller in the forward direction and a rotation angle thereof in the inverse direction formed in a predetermined time period are expressed as A and B, respectively, the supply roller driving device drives the supply roller such that a relationship of A<B is maintained. With this configuration, the mobility of the toner contained in the toner hopper may be prevented from deteriorating. Further, the amount of the toner supplied to a development roller can be kept at a constant value. Accordingly, the image forming apparatus can be reduced in size and prevent insufficient supply of the toner to an image.
There is also a related art development device which includes a development sleeve having a surface formed by a conductive resin layer which is equal in charging polarity to a developer. The conductive resin layer includes at least a joining resin, a conductive fine powder, and a charging control agent. Further, a rotation center X of a mixing member is located under a horizontal plane H which intersects a rotation center of the development sleeve.
However, it is hardly possible to stabilize and maintain chargeability and mobility of the toner of the nonmagnetic one-component developer for a relatively long time period by using the above-described techniques. In addition, there is another problematic phenomenon in which the toner has an inferior mobility when an image forming apparatus using the toner is left unused for a relatively long time period. This is because the toner is rid of air and cohesion of the toner particles is increased.